Question

What is the pH of a saturated solution of Ca(OH)2 if the solubility of calcium hydroxide is 1.50 g/L?

Answer 1

Formula: Ca(OH)₂ ⇌ Ca²⁺ + 2 OH¯

Solubility product is written as:

K_sp = [Ca²⁺] [OH¯]²

DATA: Solubility (s) = 1.50 g/L = 0.0202 M (Convert g/L into molarity by dividing g/L by molar mass of Ca(OH)₂

This is an important point: what we have calculated is 's' and it is NOT the [OH¯] for the given compound Ca(OH)₂. That value is '2s.'

Because: K_sp = [Ca²⁺] [OH¯]² = [s] [2s]²

[OH¯] = 2s = 2 x 0.0202 M = 0.0404 M

p^OH = - log (0.0404) = 1.393 [As P^OH = -log (OH^-)]

p^H = 14 - 1.393 = 12.606 [ As P^H = 14 - P^OH ]

Final answer: p^H of the saturated solution of Ca(OH)₂ = 12.606